Because to build and maintain a system that charges varying rates for a letter would be expensive, very cumbersome to use and how would you, the customer, know how many stamps to use? Then you have the "what do we do with all of the letters that didn't have enough postage"? All adds up to more costs and higher postage rates not less. FEDEX and UPS have similar options now. Ship it anywhere for the same price. Airborne Express started that.